1. Field of the Invention
The present invention relates generally to a method and system for operating of an internal combustion engine having one or more deactivatable cylinders. More particularly, the invention relates to a method and system for transitioning operation of a variable displacement internal combustion engine so as to reduce undesired engine torque responses occurring during displacement mode transitions of the engine.
2. Background Art
Variable displacement internal combustion engines have been developed to provide maximum engine torque output while operating the engine with a full complement of so-called xe2x80x9cactivatedxe2x80x9d or xe2x80x9cenabledxe2x80x9d cylinders, and to minimize vehicle fuel consumption and exhaust emissions while operating the engine with a fewer number of activated cylinders. During high speed, high load operating conditions, for example, all cylinders are usually activated as required to provide maximum torque. During low speed, low load conditions, however, individual or banks of cylinders are deactivated in order to minimize fuel consumption and reduce emissions. Variable displacement capabilities can be combined, for example with variable cam timing (VCT), to further improve the fuel economy and emissions performance of the vehicle.
A problem with conventional variable displacement engines (VDE""s), however, occurs when transitioning engine operation between various displacement modes, e.g., full cylinder mode to a reduced cylinder mode and visa-versa. During transitions, during which the number of activated cylinders is increased or decreased, the driver-demanded torque must be maintained for the transition to remain imperceptible to the driver. When transitioning from full cylinder mode to a reduced cylinder mode, for example, a powertrain control problem arises in that the manifold pressure required to maintain a constant driver-demanded torque output is different than that required in full cylinder mode. This is so because the per cylinder load changes with the number of activated and deactivated cylinders. Likewise, when transitioning from a reduced cylinder mode to full cylinder mode, a different manifold pressure is required.
Undesired torque disturbances during transitions can be minimized by properly operating an engine""s electronic throttle. A problem with such a method however is that manifold pressure cannot change instantaneously. Thus, a transition from one cylinder mode to another will cause the torque output of the engine to surge or lag the driver-demanded torque until the manifold pressure can be regulated using the electronic throttle.
A known solution to this problem is to control the electronic throttle to establish a target or adjusted manifold absolute pressure (MAP) just prior to a transition from one cylinder mode to another. After the MAP has been adjusted, designated cylinders are deactivated and the engine is placed in reduced cylinder mode. Thereby, when the engine is transitioned to the reduced cylinder mode, the engine""s intake manifold is filled as required to maintain the driver-demanded engine torque immediately upon cylinder deactivation. Similarly, when transitioning from a reduced to a full cylinder mode, the MAP is lowered to maintain the driver-demanded engine torque immediately upon cylinder activation. In either case however, the adjusted MAP still often yields an engine torque that is either in excess or below the driver-demanded engine torque.
To compensate for the adjusted MAP, spark retard techniques are used to maintain the driver-demanded torque during cylinder mode transitions. See, for example, U.S. Pat. Nos. 5,374,224 and 5,437,253 assigned to the assignee of the present invention. In the case of a transition from full to reduced cylinder mode, for example, spark retard is used to reduce engine torque just prior to cylinder deactivation. However, combustion instability introduced by the spark retard serves to limit the amount of torque reduction achievable with these techniques.
Accordingly, with a variable displacement internal combustion engine having a VCT mechanism, the inventors herein have recognized that the VCT mechanism itself can be used to more accurately control engine torque output during transitions to and from reduced cylinder mode operation of the engine.
The aforedescribed limitations of conventional control methods and systems are substantially overcome by the present invention, in which a method is provided for operating an internal combustion engine having a variable cam timing mechanism in cooperation with a plurality of deactivatable cylinders and corresponding intake valves. The method includes the steps of scheduling a transition mode of the engine, determining a desired engine torque during the transition mode, determining a VCT phase angle based on the desired engine torque, and operating the variable cam timing mechanism in accordance with the VCT phase angle to provide the desired engine torque output during the transition mode. Preferably, the step of determining the desired engine torque includes determining a desired cylinder air charge required to produce the desired engine torque. The desired air charge is then used to select the VCT phase angle required to operate the VCT mechanism to provide the desired engine torque output during the transition mode.
A corresponding system is also provided for operating an internal combustion engine having an intake manifold, an electronic throttle, an ignition system and a variable cam timing mechanism in cooperation with a plurality of deactivatable cylinders and corresponding intake valves. The system includes a manifold absolute pressure (MAP) sensor disposed in the intake manifold and a controller coupled to the MAP sensor for receiving a signal from the MAP sensor. Alternatively, one or more sensors are provided for inferring MAP. The controller includes computer program code and databases for determining an occurrence of a transition mode of the engine, determining a desired engine torque during the transition mode, determining a VCT phase angle based on the desired engine torque, and for operating the VCT mechanism in accordance with the VCT phase angle to provide the desired engine torque during the transition mode.
An advantage of the above-described method and system is that a VCT mechanism can be used to minimize the effects of undesired engine torque perturbations, fluctuations, disturbances and the like occurring during transitions between operating modes of a variable displacement engine (VDE). Specifically, by operating a VCT mechanism in accordance with the present invention, manifold air pressure can be more accurately controlled during transitions of the VDE engine from a full cylinder mode to a reduced cylinder mode and visa-versa. Dual equal variable cam timing (DEVCT) actuators, for example, can be used to control the relationship between cylinder load and manifold vacuum by varying the relative phase angle of the cam with respect to base timing to avoid undesired torque responses by the engine. When transitioning from a full cylinder mode to a reduced cylinder mode, for example, cam retard can be scheduled to reduce engine torque output when the manifold air pressure is higher than what it should be for a desired, driver-commanded torque output.
In addition, the method of the present invention can be combined with conventional spark retard techniques to provide more improved torque response without significantly impacting combustion stability.
Further objects, features and advantages of the invention will become apparent from the following detailed description of the invention taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.